1. Field of the Invention
The present invention relates to a method and apparatus for ejecting ink droplets onto a substrate and, more particularly, to a method and apparatus suitably used in a process for manufacturing a LCD device including ejecting an ink mixed with spacer members onto a substrate of an LC panel to form spacers.
2. Description of the Related Art
LCD devices have a variety of advantages including slim body, light weight and low power dissipation, and are therefore widely used as a display unit for an audio-video equipment and an office automation equipment. The LCD devices usually includes a liquid crystal (LC) panel in which a LC layer is sandwiched between a substrate (hereinafter referred to as a TFT substrate) and another substrate (hereinafter referred to as a CF substrate). In the TFT substrate, switching elements such as TFTs (thin film transistors) are arranged in a matrix. In the CF substrate, color filters (CFs) and a black matrix (BM) layer are formed. Orientations of LC molecules in the LC layer are controlled by an electric field generated between two electrodes, which are provided on one of the substrates or respective substrates, to thereby change the light transmittance of the LC layer.
In order to improve the image quality of the LC panel as described above, control of a gap (cell gap) between the TFT substrate and the CF substrate is important. Usually, spacers (such as spherical spacers or columnar spacers) having a predetermined shape and dimensions is provided between the substrates. For example, a LCD device is known in which columnar spacers are provided at fixed points, for the purpose of achieving a higher performance such as higher response speed, higher definition, and higher contrast ratio (refer to Patent Publication JP-2003-215612-A1, for example).
The columnar spacers described above are less elastic compared to typical spherical spacers. The columnar spacers therefore have the drawback that a change in the volume of LC molecules caused by a temperature fluctuation incurs a larger range of unevenness of the image due to an uneven gap or due to a larger strain caused by a thermal stress in the LC panel. Since there has been a tendency to achieve a narrower gap for the LC layer in recent years, the above drawback causes a more critical problem. In addition, the columnar spacers are formed by use of a photolithographic technique, and thus complicate the manufacturing process and raise the fabrication costs.
In view of the above drawbacks in a LCD device including the columnar spacers, there has been employed a technique using spherical spacers in the LCD device, wherein the spherical spacers are mixed with an ink ejected from an ink jet nozzle. For example, Patent Publication JP-1999-24083-A1 describes a configuration in which a number of spacer members are ejected in a single ink droplet from an ink jet head. In the described configuration, a plurality of spherical spacer members are aggregated to form a spacer, and the thus formed spacer is disposed to overlap a stripe portion of the black matrix, at which adjacent pixels are separated. The stripe portion may be at a cross section between two perpendicular stripes of the black matrix, or may be a T-shaped intersection between two perpendicular stripes.
It is desirable that the ink jet head be provided with a plurality of ink jet nozzles arranged one a line, or in a one-dimensional array, and the ink jet nozzles eject therefrom ink droplets simultaneously, so as to improve the throughput for the ink ejection. After a single ejection of the ink droplets from the plurality of ink jet nozzles, the array of the ink jet nozzles is moved in the direction perpendicular to the extending direction of the array to scan a single area of the LCD device. After completion of ejection of the single area by a scanning ejection, the array of ink ejection nozzles is moved in the direction parallel to the extending direction of the array of the ink jet nozzles, and an adjacent area is then scanned similarly to the first area, although the scanning direction is opposite to the direction for forming the previous single area.
In the above configuration of the ink jet nozzles however, it is likely that the ink droplets ejected by the plurality of ink jet nozzles do not have a uniform volume, and that the volume of the ink droplets varies monotonically from one end of the array of the ink jet nozzles to the other end of the array. This causes the volume of ink droplet significantly differs between an ink droplet ejected by an ink jet nozzle located at the one end of the array and another ink droplet ejected by another ink jet nozzle located at the other end of the array, although both the ink droplets are juxtaposed on the LC panel. The different volumes of the ink droplets cause different numbers of the spacer members included in each spacer, and causes different elasticity between adjacent stripes of the black matrix. This incurs an uneven gap length in the case of a volume expansion of the LC layer caused by a temperature rise.